Method for the isothermal treatment of alloys after casting



Feb- 24, 1959 J. N. CARTER ET AL 2,875,109

METHOD FOR THE ISOTHERMAL TREATMENT oF ALLoYs AFTER CASTING Filed Aug. 2,5, 1954 2 Sheets-Sheet 1 O OINVENTOR o g v 1 `TOI-'IN N. CARTE-' DONALD N. ROSENBLATT ATTORNEY Feb 24, 1959 J. N. CARTER ET AL 2,875,109

METHOD FOR THE IsoTHERI/IAL TREATMENT oF ALLoys AFTER CASTING Filed Aug. 25. 1954 2 Sheets-Sheet 2 wal Mmmm, mmm@ I m, m

USDOO lOxOOO W2 2 2*/2 DIAMETER 0F BARS IN INCHES Qmwmmmm INVENTOR JOHN N. CARTER l DONALD N. ROSENBLATT I BY M/ 5M ATTORNEY METHOD FOR THE ISOTHERMAL TREATMENT F ALLOYS AFTERCASTING j l John Norman Carter and Donald Norman Rosenblatt, Salt Lake City, Utah, assignors, by mesne assignments, to The Eimco Corporation, Salt Lake City, Utah, a corporation of Delaware ApplicationAngust 25,v 1954, Serial No. 452,164

` 19 claims. (c1. 14s-3) This invention relates tothe treatment of alloys and to new alloy articles produced thereby.

We have found that alloys which undergo a timedependent phase transformation at a temperature below their solidifcationpoint and a temperature-dependent phase transformation at a temperature below` that of the time-dependent transformation can be substantially improved in their metallographicstructure and in their useful properties by subjecting the alloy after casting to a substantially isothermal treatment within the temperature range in which said time-dependent phase transformation occurs before thealloy is allowed to cool to a temperature at which the temperature-dependent phase transformation occurs.

When such alloys cool from the solidication temperature certain crystallographic features are permanently established and future response of the metal to `either heat treatment or mechanical working is thereby determined at least to the extent that all future behavior must be within limits created bythe chemical composition of the alloy and the structural features imposed by the mode of cooling from the liquid statement. i The method of the invention provides desirable crystallographic structures and avoids the formation of deleterious crystallographic structures to an extent not heretofore attained or envisaged with alloys of this type. j p

The term time-dependent phase transformation is used todenotea phase transformation which can proceed to substantial completion at constant temperatures the extent of the transformation beingdetermined primarily by the-length of time during which'the alloy is `held in the temperature range in which the transformation occurs.

The term temperature-dependent phase transformation is used to denote a phase transformation which, at any given temperaturewithin `the temperature range in which the transformation occurs, proceeds to a degree determined primarily by the temperature and substantially independent of the time at which it is held at the given temperature. s i i p p 'When an alloy can undergo more than one` time-dependent phase transformation `at different temperature ranges below its solidication point, the preferred iso- 2,875,109 Patented Feb. Zf-lplti` 2 In its application to ferrous alloys, the invention contprises subjecting the cast alloy, before anyportion of the casting has cooled to temperature at which any transformation from austenitic structure to any non-pearlitic structure begins to take place, to a temperature at which pearlitic or equivalent eutectoid transformation occurs for a time suilicient to attain substantially complete pearlitic or equivalent eutectoid transformation. i

`The critical isothermal treatment at a` temperature at which the highest temperature time-dependent phase transformation occurs, prior to any cooling of the casting to a temperature at which a temperature-dependent phase transformation occurs will be designated herein by the term determinant tempering.

The determinant tempering treatment of the invention is,in general, not a substitute for, but an adjunct to, the subsequent heat treatment of the metal in accordance with `the usual practice for the various types of metals and has been found to result in propertiesv of the alloys than has been attained by, or expected from, such treatments heretofore.

Among the improvements in ferrous metals attainable by determinant tempering as dened herein are:

(l) Elimination of difficulties and defects in the processing of castings, such as cracking from initialcooling or on mechanical or flame processing, i i j (2) Elimination of difficulties and defects `in service applications, such as poor weldability, poorjmachinability, low hardenability, uneven wear patterns, and poor wear resistance, j

(3) Improvement in tensile properties, including iinpact resistance, effective yield strength, fatigue life, etc.,

thermal treatment is effected within the temperature range of the time-dependent phase transformation occurring at the highest temperature and preferably at a temperature within the range at which such transformation takes place mostrapidly. f i

Ferrous alloys are illustrative of the types oflalloys to which the principles of the invention may be applied since, as is well known, they undergo a plurality of phase transformations below the solidification point. The time-dependent phase transformation occurring at the highest temperature in the `ferrous alloys is the eutectoid transformation usually designated as the pearlite transformation.` w i i The most prominent temperature-dependent phase transformation occurring in typical ferrous alloys is usually designated as the martensite transformation.

(4) Improvement in castability of complicated parts,

j (5) Making possible the substitution of single alloy additions for multiple alloy additions with no sacrifice of properties, and j (6) Making possible more `uniform results from heat treatment. j

Other advantages limited to, or particularly important in, certain types of alloys will be more particularly pointed out hereinafter. i

The exact temperature and time of the determinant tempering treatmentis variable within ranges defined lfor each. composition by the parameters of the acceleratedY pearlite, or equivalent eutectoid, reaction rate often referred to in the case of steels and related compositions as the nose ofthe TTI diagram. Subsequent heat and Example L Chrome-moly high carbon-steels This example describes the application 'of the principles of the invention to the production of strong, tough and wear-resistant steel articles suitable for such uses as ball and rod mill liners, crusher jaws, crushing rolls and other uses requiring a combination of high hardness and high yield strength. j i

Ananalysis range suitable for the production of such articles by the method of the invention is:

Carbon .70 `to .85%.3A Manganese r- .85 to.2.00%. j S1l1con .50 to .75%.

greater improvements in useful Chromium .9o to 1.50%. Molybdenum"` .35 to .50%. Phosphorus .05% max. .Sulphur-ranma .05% max.

'The'analysis .given .above is subject to considerable -variation. However, for the purposes indicated, carbon in excess of .95 and less than .60% regardless of the balance of the analysis is undesirable.

The steel should be produced using accepted good practices in steel-making `so as to produce a killed steel properly deoxidized and amenable to the pouring of .castings in green sand molds, free from porosity and other casting .defects ywhich may originate from improper melt- .ing practices. Generally, `aluminum will provide satisfactory .deoxidization =to steels of Vthe preferred compositions.Y However, the use of calcium-silicon or calciummanganese-silicon, titanium, yor any combination of such -deoxidizers wi-t-h `or without aluminum would not of `themselves offer any diiculty Vinsofar as compatibility of residuals with the preferred compositions is concerned.

The -productionof articles from such steels will be described with particular reference to the diagram of Fig.

1, illustrating theproduction of articles such as ball mill liners. In the gure, the temperatures of heavy sections are indicated by a solid line and the temperatures of thin sections are indicated by a dashed line.

' The Vcastings are poured at (l) and before any part 'thereof fhas ycooled yto the Ms temperature, they are placed in any suitable reheating furnace at (2), heating Lto a temperature (.37) at which the rate of transformation of austenite to pearlite yis high. For the steel f this example, temperatures -wit-hin the range of about lll50 to about l275 F. are suitable. At 1250 F., the time for completion of transformation of austenite to pearlite -will -not exceed one hour.

Itwill be seen from the temperatures of thin and heavy sections between (1) `and (3) that it is advantageous, vthough not necessary, nto allow the castings to drop somewhat below the temperature of the Adeterminant tempering step after they are removed from the molds. Since the thin sections both cool and reheat faster than the heavy sections, by lcooling below, and then reheating to, 'the ldeterminant tempering temperatures, the entire casting is more quickly brought to a substantially uniform ytemperature in the determinant tempering furnace.

Upon lcompletion of the ldeterminant tempering, the castings are cooled to room temperature (4) by either furnace of or air cooling. They are structurally stable and may be stored indefinitely, if desired, at this stage ofthe manufacturing process. The normal foundry sequence of cleaning operations may be .given the castings at this point. Such procedures as shot or sand blasting, removal of gates, risers, chill nails, etc., grindingby use of snagging wheels in swing frames, chipping, reaming `of holes, and flame cutting, gouging, or washing will not adversely affect castings which have Vbeen determinant tempered at this point, nor will such operations jeopardize the attainment of the desired finished quality of the product in connection with subsequent heat treating procedures. The steel compositions of this example are not such as to permit extensive machining operations to beperformed on the castings at this stage of the manufacturing process. If it is desired to perform machining operations on .the castings, this may be done after a special heat treatment as later described herein.

` Upon completion of a normal sequence of foundry cleaning operations, the castings are ready for the second stage o f heat treatment which maybe referred to as a. homogenization treatment. This operation consists of heating the castings in any commercially suitable equipment to a temperature of from 1700v to 1900? F. and holding theV castings at the selected temperature l(5) withinthe range of 1700 to 1900vo F. for the period of several hours after the entire load of castings vhas 4 reached the required temperature and such temperature is uniform throughout all sections of all castings on the load. The holding time is of large latitude but it will be dependent on two /factorsz (l) The maximum section thickness of the heaviest sectioned casting on the load.

(2) The composition of the castings and the selected homogenization temperature.

At the optimum homogenization temperature for the cornpositicn given above, the holding time may .be safely calculated by allowing 11/2 hours per inch `for each -inch of ythe maximum section thickness of the Aheaviest Vsection casting on the load.

After the castings have `been Aheld at the required ternperature for the required time period, there are several alternate cooling programs which may ybe used depending upon the equipment available, the speed of production required, .and the necessity orlack .of necessity for making the castings micro-structurally suitable for machining operations.

When there is no `subsequent machining to be `done on the casting, the cooling ofthe castings may be accomplished any commerically practical means either very. rapidly `or very slowly, providing the coolest section of any-casting on the load is not allowed to cool below the Ms temperature f or lthe composition prior yto the `reheating for the austenitizing heat treatment which constitutes the third stage of heat treatment. Where the castings are not to immediately progress vfrom t-he homogenizing stage of .the lheat .treating cycle to `the austenitizing stage, it is necessary that no martensite be allowed to form and be ypresent .when the castings reach room temperature. This may be accomplished b y:

(l) Allowing Athe castings to .Slow Cool `in the furnace Yfrom the homogenization temperature dow-x1 to room temperature;

(2) Cooling .by any practical means `either vvery .slowly or very rapidly to a temperature of from 1275 F. to 11.50 F- and holding at :this .temperature .for a period 0f from l to 3 hours followed by .Cooling by any raras.- tical means either very rapidly Aor vvery slowly to a temperature of 70 F. or room temperature, whichever is lower.

When subsequent machining operations are ,to be performed on the castings, the cooling of the castings may proceed at any rate possible with commercial equipment down to a temperature of 1290 F. The cooling of the castings is arrested and the castings are held at this temperature for approximately l5 to 20 hours. Upon Acompletion of the holding period the castings may be cooled by any convenient means to room temperature, after which they will be micro-structurally suitable for machining. There are obvious variations in this phase ofthe homogenization treatment. Any variationis acceptable providing a spheroidized structure which will give the lowest Brinell hardness number for the steel composition under consideration results .from the .treatment selected. This machinability treatmentmay becombined with'the homogenization treatment as described above, or it may be added asa separate treatment after thecastings have been cooled from the homogenizaticn temperature according to a `suitable -cooling ,method as .described above.

Upon completion o f the homogenization treatment the castings are then ready for the third stage of heat treatment which may `b e yreferred to as thesaustenitizing treatment. Castings will enter this stage .of h eat 'treatment in any o f several conditions: f

(l) While still hot from the rapid vform of homogenization treatment;

2) At `root!) tnmperature afterone of the slow forms of homogenization'treatment;

(3) Rough machined after suitable pre-treatment;

l(4) Einishedrmachnedafter suitable .pre-treatment.

in austenitiging, the 4castings are heated 1in any-commercially suitable equipment. except-.that for a casting that has bee'n finished machined such equipment must be an atmosphere controlled furnace or a molten salt bath furnace, to a temperature of from 1450 F. to 1550 F. (6). The castings are held at the selected temperature within the range of from 1450 F. to 1550 F. for a period of several hours after the entire load of castings has reached the required temperature and the temperature is uniform throughout all sections of all castings on the load. The holding time is of large latitude, but may be satisfactorily selectedon the basis of using 1V. hours per inch for each inch of section in the maximum casting section on the load.

The selection `of the exact austenitizing temperature is somewhat flexible within the 1450 to 1550 F. range, but for steels of the compositions preferred for the present example a temperature of about l500F. will generally be the most satisfactory.`

The cooling of the castings from the austenitizing temperature is a special procedure and, therefore, is considered separately. The cooling from the austenitizing temperature constitutes the fourth stage of heat treatment of the present example and maybe referred to as the quench and hardening treatment.

In quenching, the castings arecooled (7) by submerging into a molten salt bath or lead bath of high cooling powerand suicient agitation and volume of coolant to provide as rapid as possible cooling of all sections of all castings being quenched to the bath temperature. The bath temperature is held at a selected point within the range of 400 F. to 500 F. The load being quenched should be so spaced that circulation of the quenching medium around each casting on the load is possible. The equipment used and the design of the quench tank is of wide latitude but it should, for the size ofthe load being quenched:

1) Provide a cooling power for the entire load being treated equal to an H value (heat transfer equivalent) of from .2 to .5, but no less than .2. i

- (2) Have a temperature rise for the less than 100 F. after quench.

(3) Be controllable with respect to bath temperature to a uniform plus or minus 25 F. temperature.

In the fourth stage of heat treatment `it is desirable to transfer the castings from the austenitizing furnace to the quench bath in the shortest possible handling time. Should the time be excessive, thin sections of castings or thin castings may reach a transformation, temperature prior to quenching. This would have a deleterious effect on the subsequent hardening treatment. After` entering the quench hath the castings are held in theybath until temperature equalization for all sections of all castings be ing quenched has been reached. While holding time is dependent on both load and casting geometry, ordinarily the time will not need to exceed l hour, and will often amount to less than 1/2 hour. After temperature equalization the castings are `drained of quenchant for a short time `and then forthe first` time theqcastings are permitted toibecome fully martensitc by in still air to room temperature or at least to 70 F. `The quench bath temperatureis best arrived at by using a temperature Whichisplus or minus 25 `F. `from the calculated or measured Ms of the steel composition being treated. However,` temperatures above 500 F. are likelyto result in `insufficient cooling power for the bath, and temperatures below 400 F. would not lbe found in steels of the present example. i

After the castings have cooled to room `temperature they are fully martensitic and ready for the fifth and final stage `of heat treatment. The fifth stage of heat treatment maybe referred to as a stress relief treatment. The stress relief treatment should be carriedout as soon as possible after the castings have reached room temperature in slow cooling from the bath temperature. Extending the` interval between the fourth and fifth stages of heat load being treated V.treatment ofthe present example increases the oppor- 6 tunity for stress caused Ibymartensite formation to comibine with stresses induced by handling or temperature variations and thus create triaxial stresses` which may initiate internal fissures prior to the stress relief treatment.` The purpose ofthe stress relief treatment is to allow the martensite toadjust to a more stable crystallographic lattice arrangement permitting of an axial ratio change in the direction of unity, and to relax such stresses as are present as a result of the volume changing transformation produced during hardening. The treatment may be carried out at any temperature in the range of from 400 F. to 550 F.

The treatment may be most conveniently executed by using the same molten quenching bath as used in the fourth stage of heat treatment. However, the treatment may be accomplished in any commercially suitable equipment. After every section of the castings being treated has reached the stress relieving temperature, the castings should` be held at such temperature for a period of from 1 to 3 hours (9).

A steel of the composition given above made into ball mill liners by the process outlined above had a Brnell allowing a slow cool (8) t hardness of 109 Brnell.

hardness of 606 to 609 and an elastic limit of 248,600 to 272,800 p. s. i. The same steel made into liners by the same method except that the determinant tempering at 1250 F. was omitted had a Brnell hardness of 590 to 597 and an elastic limit of 31,250 to 42,500 p. s. i.

When the determinant tempering stage of heat treatment was omitted, it was found that the structure in the center of a 6 inch section had an ASTM grain size of 6 compared to an ASTM grain size of 10 when determinant tempering was included. The bainitic hardenability `was reduced by 1.50% and the pearlitic hardenability by about 50% by elimination of the determinant tempering. t

Casting processing difficulties of the type referred to above encountered when no determinant tempering was used on the steel of this example were eliminated by `the determinant tempering treatment. t

Example 2.--Mangcznese steels Fully austenitic manganese steels containing from 4 to `10% manganese may be produced by including the determinant tempering operation of the invention in the heat treating cycle. Such steels are more suitable for many service applications than the high manganese (10 to14% Mn) Hadiield type steels heretofore used.-

For example, castings of a steel containing 1% carbon and 4% heating for about 3 to 6 hours at 1050 `to 1100 F. before any portion of the casting has cooled below 400 F. The castings are austenitized by heating at 16009 F. and quenched in water or in a molten salt bath. The resulting articles are highly strain hardenable and abrasion hardening, and maintain their strength up to 600 F. The term abrasion hardening is used herein to denote the ability of an austenitic material to transform to martensite in thin surface layers by abrading in the absence of impact. t

If it is desired to machine the articles, they may vbe made machinable by spheroidizing at 1200* F. before they are subjected to the austenitizing treatment. The 1% carbon, 4% manganese steel containing 1.36% silicon, machined into a shot blast nozzle and. `made austenitic by quenching from 1620" F. into molten salt at 450 F. shows 28 hours of running life at an initial In the same service a white iron nozzle at 450 Brnell shows a running: life of 21 hours, a martensitic steel at 740 Brnell shows a running life of 2-21/2 hours, and a martensitic steel at 590 Brnell shows a running life of 15'1/2 hours. i l When machined into a Wheelabrator abrasive wheel bladethis austenitic manganese steel at an initial hardness of 109 Brnell gives equal wear to the non-machinablc high alloy martensitic, white iron blade which has a manganese can be determinant tempered by -Brinell hardness of 652. After usegthe austenitic blade shows an abrasive 'hardness factor of 647 Brinell when measured 'by supercial hardness tests on the worn blade.

In bothof the above examples the ordinary high (Hadeld type) manganese steel is'greatly inferior due to its inability to abrasion harden.

The steel of the present example is capable of over 1.60D of Vbend at the elevated temperature of 600 F. where ordinary high manganese steel cannot be bent at this temperature. The impact strength of the 4% manganese material at 300 F. is in excess of 120 foot pounds.

Martensitic manganese steels in ranges of composition that are ordinarily too brittle to be useful, for example, .steels containing 2.5% of manganese and 0.7% of carbon,y can be made into articles of high effective strength with a hardness vof over 7.40 Brinell by subjecting the castings before cooling as low as 400 F. to determinant tempering at 1l00 to 1l50 F., and then heat treating by conventional hardening or martempering. Low alloy steels can .be given increased hardenability through higher manganese contents if determinant tempering is used to neutralize the undesirable elfects which would otherwise accompanymanganese contents in excess of 11/z%.

Example 3.-Cast irons The application of the principles of the invention Ato the production of cast 'iron articles Vinvolves particularly control `of the amount and crystallographic structure of the graphitic carbon contained therein.

The formation of graphite in any cast iron is a twostage phenomenon: first, graphite forms in gamma iron as a yrelatively tine dispersion; second, the dispersed par fieles grow immediately after v,transformation to alpha iron begins. The size and distribution of graphite particles in gamma iron ,is a function of melt chemistry only. The size and distribution of graphite particles ingal V,ha ,iron is a function of both melt chemistry `and cooling conditions from the gamma to alpha transformation temperature down to room temperature; that is, slower cooling promotes the increased growth and coarsening of graphite flake.

The present day classification of alloying elements as used in c'ast irons is a gross description and inadequate in'light of the true physical metallurgy of cast iron. When certain elements are classified as graphitizers and others as ".carbide stabilizers it is necessary to know which allotropic form of iron is indicated. In other words, some alloying elements given the gross name of graphitizermay promote graphitization in gamma iron but not in alpha iron, or just the reverse, or a combination of both. Forexample, aluminum is described in the literature as a strong graphitizer. Actually, it only promotes graphitization in alpha iron, not in gamma iron. The functions of an alloying element as to its relative influence on graphitization or stabilization in the gamma and alpha iron is directly related to the difference in solubility .of that element in gamma and alpha iron.

Graphitization of carbon in alpha and gamma iron might be expected to be a reversible reaction. That is, graphite formed in gamma iron and grown in alpha iron should go'back into Vsolution to the extent of its alpha growth when the alpha iron is returned by reheating to gamma iron. -In practice such Vis not the case. What actually happens is that the graphite continues to grow on reheating in the alpha phase and is only slightly redissolved (if at all.) in Vreheating to the gamma phase. Therefore, graphite reversibility is blocked as the initial cooling from the molten state goes beyond a certain temperature range in the alpha phase, or it may be said that the graphite has become stabilized.

'If isothermal transformation is introduced at the phase changeinfthe initial .cooling .of grey iron, the reaction ofagammairon-to pearlite will `dominate and suppress theireaction Adiganme `graphite `to alpha graphite and carbon available for deposition on existing graphite nuclei. In order to obtain the .maximum effect of isothermal transformation in grey iron, it is necessary to return any graphite growth product to solution in gamma iron before it reaches the lowest allotropic temperature change (stabilization 4temperature equivalent to Ms points in steels) before carrying out the pearlitizing treatment.

The addition of alloying elements depresses the critical temperature range as in the case of steels. lIn general, the equivalent ina cast iron of the Ms ktemperature of steel is determined by the composition of the non-graphitic matrix.

The effect of the determinant tempering treatment of the invention is to reduce the amount of graphitic carbon and greatly to increase the uniformity of all constituents regardless of varying `dimensions of the cast article. f

A casting having graduated sections of -1/2, 3A", 1", 1%", 21/2", and 4" poured in a gray iron lcomposition of 3.10% total carbon, .64% manganese, 2.70% silicon, .23% phosphorus and .093% sulphur, when determinant tempered by reheating to 1600 F. before reaching 800 F. in cooling from the solidiiication temperature and then lowering to l250 F. and holding for Zhours gives a Brinell .hardness of 183 .in all section steps, and a uni- 'fornr pearlitic structure having a tensile strength of 31,000 p. sfi. rll`he s ame casting poured from the Asame ladle without determinant tempering gives a range of Brinells in different sections of from 1 35 to 178 and has a tensile strength in the -`1 section of 20,500 p. s. i.

Similar castings produced from a composition of 3.30% total carbon, .89% manganese, 2,36% silicon, .30% phosphorus, and .090% sulphur, show a uniform Brinell hardness in all sections of 212 BHN and a. tensile strength of 33,000 p. s. i. when determinant tempered. 'Where no determinant tempering is used the hardness varies from 127 to 183 Brinell and the tensile Strength is 22,000 p. s. i. in a 1" section. By subject ing castings of an iron containing 3.6% carbon, 1.3% silicon and 2.41% manganese to determinant tempering in the range ofA 1275 to 1325 F. before any part of the castings has cooled below about 600 F. and thereafter quenching from about 1600 F., a martensiticaustenitic white iron casting is obtained. This alloythas a uniform matrix structure of line martensite and metastable austenite `coupled with a uniform dispersion of primarycarbide. M artempered the material is extremely tough and shows 555 Brinell. Under a 3000 kg. load this iron hardens to 627 BHN. y

Martensitic grey iron articles Vmay be produced by subjecting castings of -an iron containing 3.5% carbon, 1.3% silicon and 0.37% manganese to the treatment described in the preceding paragraph. Thus an ordinarily mottled iron analysis can be made into both a machinable and heat treatable iron of excellent properties.

The cast irons of the present example illustrate the wide application of determinant tempering to the production of -all ltypes of Acast iron. Y

`Example lL--Oxialaion resistant steel Articles of good hardness and excellent high temperature resistance can be made of medium carbon,i high chromium steel compositions by the method of the invention.

For example, castings `of a .47% carbon, V6% chromium steel are subjected to determinant tempering by heating for about two hours at l250 F. before any portion of the castings has cooled below 400 F.; thereafter thecastings are normalized at about 1 800 Rand quenched after reheating ,in a molten salt bath. Y

`As an illustration of the eiect of determinant tempering on austenitizing temperature, when this steel is ing the determinant reheated to 150051?. the metal which has been determinant tempered will upon salt quenching'provide 450 Brinell ina 2 section, the same article ofidentical analysis provides only 220 Brinell when identically treated. i

Example 5 .-Low alloy steels` The determinant tempering treatment of the invention confers both higher and more uniform strengths and hardenability on low alloy engineering steels of the type of SAE 4140. l

For one treatment of the invention, the cast steel is heated at about 1200 F. for two hours before any portion of the castings has cooled below the `Ms temperature, which for SAE 4140 steel is about 625 F.

Such castings when given the usual heat treatment for steels of this composition typically have a hardness of the order of 250 BI-VIN and tensile strengths of the order of 125,000 p. s. i. When the treatment includes the determinant tempering of the invention, a hardness of the order of 270 and tensile strengths of the order of` 140,000 p. s. i. are obtainable. Particularly notable is the increased uniformity of hardness and strength in the thicker sections. This is illustratedjin Fig. 2 showing the hardness and strengths through cross sections of round bars of various diameters when the treatment did and did not include the determinant tempering of the invention, Vthe treatments being otherwise identical.

Castings of SAE 4140 steel which, because of intricate design, were cracked in the as-cast condition, have been successfully produced free from cracks by includtempering treatment.

Example 6.-Alumnu=mmagnesium alloy The aluminum-magnesium alloy containing about %,magnesium, in spite` of its many desirable properties, is limited in commercial use by certain disadvantages related to its high magnesium content, particularly the as-cast brittleness of the alloy. Castings of the alloy are quite brittle prior to heat treatment because of a grain boundary network of Mg5--Al3 formed during solidiiication. This embrittling grain boundary is usually eliminated by a solution treatment after the castings reach room temperature. In the handling of the castings from the molds to the various stages of production prior to the solution heat treatment, `the castings are extremely brittle and a high scrap loss is experienced in commercial operation. This diiculty can be eliminated by the determinant tempering treatment of the invention comprising heating the castings, before any portion thereof has cooled below about 775 F. at a temperature of about 825 F. for a substantial period of time.

Advantageously the molds for the aluminum-mag'- nesium castings are placed on oven cars and after casting, the cars are run into a furnace at 825 F. and held at that temperature for 16 hours. After cooling, the castings determinant tempered in this manner have the same physical properties as castings which have been cooled to room temperature and thereafter solution treated in the usual manner.

The terms casting and cast metal are used herein in their broadest sense and include ingots and, in general, any metal articles, the production of which includes the solidification of metal from the molten state.

We claim:

l. The improvement in the production of articles of alloys which undergo a time-dependent phase transformation at a temperature below their solidiication point and a temperature-dependent phase transformation at a temperature below that of the time-dependent phase transformation which comprises subjecting the alloy after casting to a substantially isothermal treatment within the temperature range in which said time-dependent phase transformation occurs most rapidly before any portion without determinant tempering.

of -the castalloy has cooled to the highest temperature at which a temperature-dependent phase transformation occurs. i i 1 `2. The improvement in the production of articles of alloys which undergo a time-dependent phase transformation at a temperature below their solidifcation point and a temperature-dependent phase transformation at a temperature below that of the time-dependent .phase transformation which comprises subjecting the alloy after casting to a substantially isothermal treatment within the temperature yrange in which the time-dependent phase transformation occurring at highest temperature occurs before any portion `of the cast alloy has cooled to the highest temperature at which a temperature-dependent phase transformation occurs.

3. The improvement in theproduction of ferrous metal articles which comprises subjecting a cast ferrous metal to a substantially isothermal treatment-within the temperature range in which the time-dependent phase transformation -occurring `at highest temperature occurs most rapidly before any portion of the casting has cooled to the highest temperature at which a temperature-dependent phase transformation occurs. i

4. The improvement in the production of ferrous metal articles `which comprises subjecting a cast ferrous metal, before any portion of the casting has coo-led to a temperature at which transformation from austenitic structure to non-eutectoi-d structure begins to takeplace, to a temperature at which eutectoid transformation occurs for a time suilicient to effect substantially complete eutectoid transformation.

5. The improvement in the production of ferrous metal articles which comprises .subjecting a cast ferrous metal, before any portion of the casting has cooled to a temperature at which transformation from austenitic structure to non-pearlitic structure begins to take place to a temperature at which pearlitic transformation occurs for a time suicient to effect substantially complete pearlitic transformation.

6. The improvement in the production of ferrous metal articles which comprises subjecting a cast ferrous metal, before any portion of the casting has cooled to a temperature at which transformation from aus'tenitic structure to non-eutectoid structure begins to take place, to a v temperature Within the range at which most rapid eutecfor a time sullicient to effect;V

toid transformation occurs substantially complete eutectoid transformation. j

7. The improvement-in the production of ferrous metal articles having a composition of .60% to .95% carbon, .85 to 2.00% maganese, .90 to 1.5% chromium, .35 to .50% molybdenum, less than .05% phosphorus and sulphur, and .50 to .75% silicon, the balance substantially all iron which comprises heating a casting of said composition, before any portion thereof has cooled below 400 F., to a temperature of from 1150 F. to 1275 F. for from one to three hours.

8. The improvement in the production of ferrous metal articles having a composition of .60% to .95% carbon, .85 to 2.00% manganese, .90.to 1.5% chromium, .35 to .50% molybdenum, less than .05% phosphorus and sulphur, and .50 to .75% silicon, the balance substantially all iron which comprises heating a casting of said composition, before any portion thereof has cooled below 400 F., to a temperature of 1250 F. for at least one hour.

9. The method of producing ferrous metal articles which comprises heating a casting having a composition of .60% to .95% carbon, .85 to 2.00% manganese, to 1.5% chromium, .35 to .50% molybdenum, less than .05% phosphorus and sulphur, and .50 to .75% silicon,

the balance substantially all iron to a temperature of ast/5,1206

temperature of from 1700* F. to 1900 F. for a period of 1172 hours per maximum casting section thickness in inches, cooling to below 1000 F reheating to a temperature of from 1450 F. to 1550 F. for a period of about 1 hour per maximum casting section thickness in inches, and quenching in a molten salt bath having an H value no lower than .2, and at a temperature of from 400 to 500 F. for a period of time sufficient to equaliZe the temperature of all sections of the casting, allowing the casting to cool in still air to room temperature, reheating to a temperature of from 400 F. to 500 F. for from 1 to 3 hours, and thereafter cooling the castings to room temperature.

' 10. The improvement inthe production of manganese I steels containing from about 1.5% to about of mangan'es'e which comprises subjecting a casting thereof, before any portion of the lcasting has cooled below the highest temperature at which a temperature-dependent phase transformation occurs, to a temperature within the range in which the time-dependent phase transformation occurring at highest temperature occurs most rapidly.

11. The improvement in the production of manganese steels containing about 1% carbon and about 4% manganese which comprises subjecting a casting thereof, before any portion of the casting has cooled below 400 F., to a temperature of from about 1050 to about 1l00 F. for about 3 to 6 hours.

12. The improvement in the production of cast iron articles which comprises heating a cast iron casting, be-

fore any portion of the casting has cooled below the highest temperature at which a temperature-dependent phase transformation occurs, to a temperature within the range in which the time-dependent phase transformation occurring at highest temperature occurs most rapidly. 13. The improvement 4in the production of cast iron articles which comprises heating a cast iron casting, b'efore any portion of the casting has cooled to the lowest temperature at which allotropic change of iron in the composition thereof can occur, at a temperature within the range at which pearlitic transformation occurs for a time sufficient to effect substantially complete pearlitic transformation.

l14. The improvement in the production of cast iron articles which comprises heating a cast iron casting, before any portion of the casting has cooled below the graphite stabilization range of the composition, to a tern- `12 peiature of `about 1250 F. until pearlitic transformation is substantially complete.

15, .The vimpro"vement in the production of cast iron articles which comprises heating a cast iron casting, before any portion of the casting has cooled below the graphite stabilization range of the composition, to a temperature of about 1250 F. until pearlitic transformation is substantially complete, cooling the casting `to room temperature and thereafter quenching the casting from abouto F. l

16. The improvement in the production of aluminummagnesium alloys having a magnesium content of about '10% which comprises heating a casting of the alloy, before any portion of the casting has cooled below about 775 F., to a temperature of about 825 F. for about 16 hours.

17. The improvement in the production of articles of alloys which comprises subjecting a cast alloy article to a substantially isothermal treatment within the temperature range in which a time-dependent phase transformation occurs most rapidly before any portion of the casting has cooled to the highest temperature at which a temperature-` dependent phase transformation occurs.

18. Thek improvement in the production of ferrous metal articles which comprises subjecting a cast ferrous metal article to a substantially isothermal 'treatment within the :temperature range in which a timer-dependent phase transformation occurs most rapidly before any portion of the casting has cooled to the highest temperature at which a temperature-dependent phase transformation occurs.

19. The improvement in the production of ferrous metal articles which comprises subjecting a cast ferrous metal article to a substantially isothermal treatment within a temperature range in which a time-dependent phase transformation of austenite occurs before any portion of the casting has cooled to a temperature at which substantial transformation of austenite to martensite occurs.

References Cited in the tile of this patent NITED STATES PATENTS 1,915,158 Fahrenwald June 20, 1933 2,219,32-0 Jones Oct. 29, 1940 21,646,375 Sefing July 21, 1953 2,661,283 Smalley Dec. 1, 1953 2,662,010 Ahles Dec. 8, :1953 

1. THE IMPROVEMENT IN THE PRODUCTION OF ARTICLES OF ALOYS WHICH UNDERGO A TIME-DEPENDENT PHASE TRANSFORMATION AT A TEMPERATURE BELOW THEIR SOLIDIFICATION POINT AND A TEMPERATURE-DEPENDENT PHASE TRANFORMATION AT A TEMPERATURE BELOW THAT OF THE TIME-DEPENDENT PHASE TRANSFORMATION WHICH COMPRISES SUBJECTING THE ALLOY AFTER CASTING TO A SUBSTANTIALLY ISOTHERMAL TREATMENT WITHIN THE TEMPERATURE RANGE IN WHICH SAID TIME-DEPENDENT PHASE TRANSFORMATION OCCURS MOST RAPIDLY BEFORE ANY PORTION OF THE CAST ALLOY HAS COOLED TO THE HIGHEST TEMPERATURE AT WHICH A TEMPERATURE-DEPENDENT PHASE TRANSFORMATION OCCURS. 